(A) 3-HBA and 4-HBA catabolism via the gentisate central pathway (continuous line). The substrates and products are: 3-hydroxybenzoate (1); gentisate (2); maleylpyruvate (3); fumarylpyruvate (4); pyruvate (5); fumarate (6) and 4-hydroxybenzoate (7). The enzymes are MhbM (3-HBA 6-hydroxylase), MhbD (gentisate 1,2-dioxygenase), MhbI (maleylpyruvate isomerase) and MhbH (fumarylpyruvate hydrolase). The gene encoding the 4-HBA 1-hydroxylase is unknown. (B) 4-HBA catabolism via protocatechuate (dotted line). The substrates and products are: 4-hydroxybenzoate (7); protocatechuate (8); 3-carboxy-cis,cis,-muconate (9); 4-carboxymucolactone (10); β-ketoadipate enol-lactone (11); β-ketoadipate (12); β-ketoadipyl-CoA (13); Acetyl-CoA (14); Succinyl-CoA (15).The enzymes are: PobA (4-hydroxybenzoate 3-monooxygenase), PcaGH (protocatechuate 3,4-dioxygenase), PcaB (3-carboxymuconate cycloisomerase), PcaC (4-carboxymuconolactone decarboxylase), PcaD (β-ketoadipate enol-lactone hydrolase), PcaJI (β-ketoadipate:succinyl-CoA tranferase) and PcaF (β-ketoadipyl-CoA thiolase).